Improved Water Vapor Sensor Takes to the Skies

June 15, 2005

BOULDER—By pairing a sleek new air sampler designed at the University
Corporation for Atmospheric Research (UCAR) with a diode laser from
SpectraSensors, Inc., researchers have hit on a technology that can
capture highly accurate atmospheric water vapor data during routine
commercial flights. The data will benefit researchers and forecasters,
who need more frequent, accurate measurements at various altitudes
worldwide to improve weather forecasts and monitor climate change.

This month UPS is flying the Water Vapor Sensing System II (WVSS II) on
25 of its Boeing (B-757) aircraft based in Louisville, Kentucky, to
compare the data to measurements from weather balloons, satellites, and
other instruments and to evaluate its performance aboard commercial planes.

Currently water vapor data is gathered by an older style of sensor using
a thin-film capacitor. These sensors are launched on weather balloons
every 12 hours from stations around the country. Satellites also gather
water vapor data, but at low vertical resolution. The WVSS II aboard
commercial flights will gather data more often, at higher vertical
resolution, and at lower cost than satellites and balloons.

"Water vapor sounds boring," says recently retired UCAR scientist Rex
Fleming, who designed the innovative air sampler, "but it's essential to
almost everything that happens in the atmosphere." Better water vapor
data from around the U.S. and the world can improve forecasts of
thunderstorms, microbursts, turbulence, fog, ceiling visibility,
rotating wakes from other aircraft, snow and ice storms, and year-round
precipitation, he says.

Water vapor also plays an important role in small storms that develop
quickly and wreak havoc with airline schedules and safety. The Federal
Aviation Administration estimates these storms can cost the aviation
industry more than $1 billion annually.

Improved aviation weather forecasts can make flying safer, allow
airlines to expand the number and location of routes, provide alternate
landing options, and save fuel. Over the long term, the new data can
verify computer model projections of climate change, which indicate
water vapor steadily increasing in Earth's atmosphere. As a greenhouse
gas, water vapor is 10 times more potent than carbon dioxide and its
increase is a key factor in the rising global temperatures appearing in
the models.

The aerodynamic design of the air sampler removes most ice crystals,
particles, rain, and other distractions to improve the sensitivity of
the measurement. (Photo courtesy Rex J. Fleming.)

The FAA certified the WVSS II for commercial aircraft flights last
December. Preliminary results show the WVSS II data are highly
consistent with the balloon data up to 35,000 feet. This month's tests
should lead to verification of the sensing system for other uses by
forecasters, air traffic controllers, and research scientists.

Mounted flush on the side of 25 UPS
(B-757) aircraft, the air sampler produces little in-flight drag.
(Photo courtesy UPS.)

"In a typical year, more water in the form of vapor and clouds flows
over the dry state of Arizona than flows down the Mississippi River,"
says Fleming. "Yet we have not had a sensing system to collect accurate
water vapor data frequently enough to be really useful for forecasts."
Commercial aircraft can fill a critical gap in atmospheric observations
by gathering accurate data throughout the global atmosphere, he adds.

Mounted flush on the outside of the plane, Fleming's sampler channels
air into the measurement cell housed in a casing the size of a cigar box
just inside the aircraft shell. The sampler weeds out most ice
crystals, particles, rain, and other distractions to improve the
sensitivity of the measurement. The laser frequency itself sees only
water vapor in the air flow.

UPS has provided wind and temperature data to meteorologists from more
than half its air fleet since 1994. In 1997, UPS added water vapor
information, expressed as relative humidity, from a first-generation
test sensor installed on 30 aircraft. The new second-generation sensors
are expected to be far more accurate and reliable, especially at higher
altitudes and colder temperatures.

Southwest Airlines will begin flying the system when further government
funds are available. The German Weather Service is in the process of
certifying the sensor, and Lufthansa will be installing four units on
commercial flights later this year. New Zealand, Australia, and South
Africa will collaborate with the German Weather Service on an initial
purchase of ten units.

The FAA's Aviation Weather Research Program and NOAA's Office of Global
Programs funded development of the WVSS II. The diode laser cell was
designed by Randy May of SpectraSensors, the manufacturer of the product.

The National Center for Atmospheric Research and UCAR Office of Programs are operated by UCAR under the sponsorship of the National Science Foundation and other agencies. Opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any of UCAR's sponsors.